Function parameters referenced in closure not properly handled by no-unused-vars

[burnnat]

Consider the following javascript:

var doStuff = function(fn) {
    fn();
};

var callStuff = function(first, second) {
    doStuff(function() {
        console.log(second);
    });
};

callStuff('foo', 'bar');

Running ESLint on this sample code yields an error from the no-unused-vars rule:

sample.js: line 5, col 25, Error - first is defined but never used (no-unused-vars)

1 problem

Normally, the no-unused-vars rule will ignore any function parameters that precede a "used" parameter. But this seems to break when the parameter is used inside a closure, as in this example - even though the second parameter is properly detected as being used, the first parameter is not ignored and causes a spurious error.

[nzakas]

You're totally correct. Looks like the closure is confusing the rule.

[nzakas]

I think this is tied to the markIgnorableUnusedVariables() only looking up one level, to the nearest function. @ilyavolodin?

[ilyavolodin]

I'm not 100% sure, markIgnorableUnusedVariables was added by somebody else. But yes, based on the code I'm pretty sure that's due to findFirstAncestorThatIsFunctionExpressionOrDeclaration() function. I think that this function should be removed and instead, we shouldn't use ancestors in markIgnorableUnusedVariables but use usedVariable.node to find function. If direct parent of usedVariable.node is function, the we need to mark all previous arguments as used, if not, don't do anything.

If nobody else will be able to get to it soon-ish, I'll try to fix it at some point (sorry, still don't have enough free time).